CONTRASTS IN STREAM-AQUIFER INTERACTIONS DRIVEN BY HYDROGEOLOGY AND GEOMORPHOLOGY

Stream-aquifer interactions differ considerably among streams in Kansas depending on the hydrogeology and geomorphology of the system. End-members range from systems with well-connected vertical and lateral flow between the stream and aquifer in unconfined sands and gravels to systems with very low flow between stream and aquifer in unconfined silts and clays in shaly bedrock. The Arkansas River in south-central Kansas is an example of a well-connected system in which increases in river flow, as well as precipitation recharge on sand bars and dry channels, moves rapidly into the alluvial aquifer. Diurnal river variations during low flow occur in response to phreatophyte pumping in the riparian zone. Relative changes in water levels in the alluvial aquifer in response to river flow are partially controlled by channel entrenchment during the last half century. In contrast, poorly-connected systems (relatively small amounts of interaction) occur in stream stretches in silt and clay alluvium deposited in valleys cut into nearly flat-lying shales and limestones just 180 km to the east. Here the valleys have greater topographic relief and recharge entering in the upland and flowing laterally through limestone units is confined by the alluvium in the floodplain, except where the stream cuts through exposed limestone. The piezometric surface in the bedrock and at the base of the alluvium at a study site in one of these streams is about 0.5 m above that of the water table in the alluvium and the stream level during low flow. Large water-level fluctuations in the bedrock and at the base of the alluvium occur within 15-30 minutes after pumping begins in small capacity wells 0.4 km away. No response to this pumping is observed in shallow water-table wells screened in the alluvium in zones of coarse gravel embedded in clay. Bedrock and alluvium base wells at the site show relatively rapid response to stream-flow variations as expected in a confined system. In contrast, the water-table wells do not show this rapid stream-aquifer response during low to moderate stream flow. However, they do show diurnal fluctuations in water levels related to transpiration from vegetation in the riparian zone; these fluctuations are also recorded by shallow soil moisture sensors.